Intensive research by many laboratories, including work performed under this grant, has advanced the hypothesis that altered processing of the B-amyloid precursor protein (APP) or decreased clearance of its amyloid B-peptide (AB) fragment are critical pathogenic events in Alzheimer' s disease. Nevertheless, despite a wealth of reports about in vitro and in vivo activities of the precursor and its derivatives, the fundamental physiological function of APP has not been established. Based on recent studies in our and other laboratories and extensive preliminary data, we propose to elucidate systematically the function of APP and the molecular mechanism by which it accomplishes this function. The central hypothesis motivating this application is that the remarkably similar proteolytic processing of APP and the Notch receptors suggests that APP is itself a cell-surface receptor with a cognate ligand, the binding of which enables the release of the cytoplasmic domain (AICD) to the nucleus to activate specific genes. Based on substantial experience in APP processing and membrane protein biology obtained during the past 24 years of this grant, we will carry out 4 interrelated Aims: 1. to purify, identify and characterize a specific protein ligand(s) for the APP ectodomain; 2. to localize the APP intracellular domain (AICD) to the nucleus and study its stabilization and activites there; 3. to assess the phenotypic consequences, including for APP processing and intracellular AB generation, of signaling by the physiological ligand(s) in intact neurons; and 4. to establish the in vivo biochemical and electrophysiological effects of APP signaling in living animals. We will compare the ligand binding and potential signaling properties of APP to those of APLP-1 and APLP-2 and also explore whether APP processing and signaling alter those of Notch. Intriguing preliminary findings about AICD nuclear localization and stabilization by Fe65 and about transcriptional changes in the brains of APP knockout mice encourage us to pursue these aims. The results of the multifaceted experimental approach described herein should be revealing not only for the normal biology of APP but also for how its regulated signaling contributes to the pathogenesis of Alzheimer's disease and to the progressive accumulation of AB that accompanies normal brain aging in humans and other primates.